1. Field of the Invention
The present invention relates to a polarizer by which light leakage based on changes of polarizing element axes caused by a change of a viewing angle in between polarizing elements disposed in the form of crossed-Nicol can be prevented in a wide-range visible light region to thereby achieve liquid-crystal display of a wide viewing angle, or the like.
The present application is based on Japanese Patent Application No. 2000-330069, which is incorporated herein by reference.
2. Description of the Related Art
In polarizing elements disposed in the form of crossed-Nicol, there was a problem that light leakage might occur when the azimuth was changed to an oblique one even in the case where the light could be cut off ordinarily in a normal-line (frontal) direction. This was because the relationship in crossed-Nicol optical axis is displaced or collapsed between the polarizing elements due to the change of the apparent angle caused by the oblique view. As a background-art device to solve the light leakage problem caused by such an azimuth angle, there was known a polarizer in which a transparent protective film exhibiting birefringence with a retardation of from 190 to 320 nm and with Nz (which will be described later) of from 0.1 to 0.9 was disposed so that the slow axis of the transparent protective film is parallel with the absorption axis of a polarizing element (see Unexamined Japanese Patent Publication No. Hei. 4-305602).
The background-art polarizer was provided to compensate for the displacement in absorption axis or the like between polarizing elements due to the change of the viewing angle as follows. As a transparent protective film to be bonded to one or each of opposite surfaces of a polarizing element for improving durability against penetration of moisture or the like, a film exhibiting retardation characteristic of about a half wavelength with respect to visible light was used instead of an isotropic transparent protective film constituted by a triacetylcellulose (TAC) film or the like exhibiting little birefringence. There was, however, a problem that the compensating measure could not cope with wavelength dispersion.
That is, wavelength dispersion, which is a phenomenon that the retardation varies in accordance with the wavelength, generally occurs in a retardation film. Hence, the function of the retardation film as a half-wave plate works only for light with a specific wavelength. For light with the other wavelengths, the retardation film cannot function as a half-wave plate accurately, so that the light with the other wavelengths is inferior in the characteristic of linearly polarized light. There therefore arises a coloring problem. Incidentally, when the characteristic of the retardation film is optimized to compensate for light with a wavelength near to 550 nm exhibiting the maximum luminous efficacy, light with the other wavelengths is colored with blue because the condition for the light is displaced from the aforementioned optimizing condition. Hence, when the retardation film is applied to a liquid-crystal display device or the like, the coloring problem reveals itself as a problem in deterioration of neutral characteristic of display.
An object of the present invention is to develop a polarizer in which light leakage hardly occurs while coloring owing to wavelength dispersion hardly occurs to thereby achieve excellent neutral characteristic even in the case where polarizing elements disposed in the form of crossed-Nicol are obliquely viewed at an azimuth displaced from the optical axis thereof.
According to the present invention, there is provided a polarizer constituted by: a polarizing element; and at least one transparent protective film constituted by two layers of retardation films with an in-plane retardation in a range of from 190 to 320 nm with respect to light having a wavelength of 550 nm, the transparent protective film being bonded onto one of opposite surfaces of the polarizing element so that a slow axis of each of the retardation films is parallel with an absorption axis of the polarizing element, the two layers of retardation films being constituted by a combination of a retardation film with Nz of from 0.8 to 0.95 and a retardation film with Nz of from 0.55 to 0.7 in the condition of nx greater than ny and Nz=(nxxe2x88x92nz)/(nxxe2x88x92ny) in which nx and ny are in-plane refractive indices of the retardation films respectively, and nz is a refractive index in a direction of thickness of each of the retardation films.
According to the present invention, there can be obtained a polarizer which exhibits a compensating function for canceling the change of an optical axis such as an absorption axis of a polarizing element by changing an optical axis such as a slow axis of each of retardation films constituting a transparent protective film in accordance with the change of a viewing angle so that both light leakage and wavelength dispersion of retardation can be suppressed not only at a viewing angle parallel with the optical axis of polarizing elements disposed in the form of crossed-Nicol but also at a viewing angle displaced from the optical axis to thereby achieve excellent neutral characteristic (colorlessness) and make linear polarizing characteristic unchangeable. The use of the polarizer permits the formation of a liquid-crystal display device, or the like, excellent in display quality such as high contrast ratio at a wide viewing angle. In addition, the polarizer is excellent in reduction of thickness and weight because the two layers of retardation films serve as a transparent protective film.